1. Field of the Invention
This invention relates to a method and apparatus for the manufacture of fiber reinforced structures. More specifically, the present invention relates to a method and apparatus for the fabrication of composite structures wherein tooling and a delivery head are employed for the placement of discrete, elongated fiber elements in mutually superimposed relationship to define reinforcement or stiffening members on fiber elements for the interiors or exteriors of composite shells and the consolidation of the fibers to form the cured composite structure.
2. State of the Art
It is desirable to have inexpensive, strong, lightweight, easily manufactured, dimensionally accurate components in a variety of sizes and geometries for use in aircraft and aerospace applications. Composite reinforced, or "grid-stiffened", shell structures, such as shrouds, casings, fuel tanks, airfoils, or wing skins, and fuselage panels, provide recognized advantages in aerospace applications over conventional metal assemblies, typically of aluminum or titanium, in terms of relatively lower weight and higher strength for the composite structures. However, meeting such criteria for components is difficult. The acceptance of all-composite structures has been hampered by the lack of demonstrated, repeatable, and inexpensive fabrication methodology and apparatus for effecting such in an automated manner. Composite structures have been found in high-performance military aerospace applications, where composite structures are formed in an non-automated manner at great expense. In non-aerospace applications, composite structures are more limited to applications where they can be formed as simple structures on existing machines. However, it is desirable to form complex reinforced composite structures for a wide variety of applications which are price competitive with metal structures and have lower weight and equal or better strength than such metal structures.
For example, commercial aircraft are typically powered using turbofan type engines. A turbofan type engine includes a ducted fan, a large diameter, axial-flow multi-stage compressor, as the primary source of thrust by the engine while the gas generator portion of the engine provides a smaller amount of the engine's thrust. Each stage of the ducted fan includes a number of fan blades attached to a rotating fan disc or hub to compress air, the compressed air flowing from the fan and expanding through a nozzle to provide thrust to move the aircraft. Depending upon the size of the engine, the diameter of each stage of the ducted fan may be approximately one meter to several meters or more in diameter and rotate at several thousand revolutions per minute. Each fan blade attached to a fan disc or hub is a highly stressed structure due to the forces acting on the blade from compressing the air flowing therearound and from the centrifugal forces acting on the blade during rotation of the engine.
Since weight is of concern in aircraft engines, it is desirable to provide the lightest engine possible to meet the operational criteria for the aircraft while providing the required aircraft operational safety. One of the desired operational safety characteristics for a turbofan aircraft engine is that if a fan blade catastrophically fails during engine operation, the blade or pieces of the blade be contained or caught within the fan housing structure to prevent damage to the aircraft, its cargo, and the surrounding engine and aircraft environment. Typically, aircraft manufacturers have required that the fan housing be such a structure for the engine, thereby making the fan housing one of the heaviest engine components.
The design of an inexpensive, strong, lightweight, easily manufactured, dimensionally accurate fan housing in a variety of sizes and geometries for use in aircraft is a formidable task. For instance, the fan housing must be strong enough to contain the energy of a fan blade when the failure occurs at maximum engine speed, must be dimensionally accurate over a range of engine operating conditions, must be easily manufactured at a reasonable cost, must be lightweight, etc. Typically, fan housings have been metal structures using a variety of reinforcing grids, typically formed of metal. However, such fan housings are expensive, are difficult to manufacture, require extensive tooling to manufacture to close tolerances, and are heavy.
In other instances, some fan housings have been composite type structures including metal components and non-metallic or organic type reinforcing components in an attempt to provide a high-strength, lightweight structure capable of containing a broken fan blade. However, such composite type structures are difficult to construct because the reinforcing structure of non-metallic materials for the fan housing has been difficult and expensive to construct. Typically, such a non-metallic reinforcing structure has employed an isogrid type structure which is difficult to reliably fabricate in quantities. The isogrid type structure is efficient in providing reinforcement for the fan housing and the ability to catch a broken fan blade while maintaining its strength and integrity even with a portion missing or broken. A composite isogrid structure may require internal or external reinforcing elements or stiffeners, ribs, adjacent a continuous shell structure, to provide enhanced stiffness to the shell structure in terms of torsional and bending resistance. The larger the shell structure, the greater the reinforcing requirement. The reinforcing elements may be discrete and remote from each other or, preferably, are in a grid structure. One favored grid structure is an isogrid of reinforcing elements at angles of approximately 60.degree. with respect to an adjacent element.
Typically, such composite isogrid structures have been fabricated by hand by applying resin (epoxy) impregnated fiber element "tows" in a grid-like pattern using soft, imprecise tooling of wood, resilient materials, etc. which affects the isogrid structure's repeatability in manufacture, dimensional tolerance variation, structural integrity, cost, etc. A number of tows are typically laid-up on a mandrel or other tooling in vertically superimposed, or stacked, relationship to define each rib of the grid. The tows are then cured simultaneously under heat and pressure with a contiguous composite shell. However, such a process is not repeatable and the product not reproducible. Alternately, stiffeners may be fabricated by automated application, or "winding", of the fiber elements in the form of continuous filaments onto a cylindrical mandrel. However, filament winding has exhibited perceptible deficiencies in terms of inaccuracy of fiber placement. as well as compaction problems of the placed fiber. Also, the filament winding generates an excess of fiber scrap since it requires a continuous turnaround path when each end of a mandrel is reached; the filament turns around at the ends of the mandrel do not form part of the final structure, and, so, are cut off and discarded. Filament winding techniques provide no capability to "steer" the fiber filament to accommodated desired variations from a preprogrammed path to place fiber on complex geometry mandrels, including those exhibiting concave exterior portions, or to terminate fiber element placement at a target point on tooling and restart the application of a new fiber element at a new target point. Filament winding has particularly severe limitations where stiffening members cross or intersect, due to the inability to eliminate or reduce fiber element build-up at the nodes where fiber elements are oriented in two or more directions cross. Furthermore, filament winding techniques lack the capability to place fiber at a zero degree angle, i.e., parallel, to the longitudinal axis of rotation of the mandrel. Therefore, a need exists for a method and apparatus for the fabrication of composite structures, such as an isogrid structure, to maintain the integrity, reliability, repeatability of manufacture, dimensional control, and cost of the structure. A need exists for an apparatus capable of placing discrete fiber elements in desired lengths and at desired angles along specified paths onto tooling so as to form stiffening structures onto which a blanket of composite fibers may be laid up to form the desired structure and the fibers cured to yield the desired structure.